Build wasm module

[jcfr]

@thewtex Based on your experience with itk-js, could you point us to relevant documentation/example to make this happen.

Here are the emscripten options we currently use:

dcmjs/CMakeLists.txt

Lines 85 to 99 in fea79b9

	set(CMAKE_C_COMPILER "${EMSCRIPTEN_ROOT_PATH}/emcc")
	set(CMAKE_CXX_COMPILER "${EMSCRIPTEN_ROOT_PATH}/em++")
	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-warn-absolute-paths -s ALLOW_MEMORY_GROWTH=1 -s ASSERTIONS=1 --memory-init-file 0")
	set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-warn-absolute-paths -s ALLOW_MEMORY_GROWTH=1 -s ASSERTIONS=1 --memory-init-file 0")
	# Emscripten toolchain options
	set(EMSCRIPTEN_TOOLCHAIN_OPTIONS
	-DCMAKE_MODULE_PATH:PATH=${EMSCRIPTEN_ROOT_PATH}/cmake/Modules
	-DCMAKE_TOOLCHAIN_FILE:FILEPATH=${EMSCRIPTEN_ROOT_PATH}/cmake/Modules/Platform/Emscripten.cmake
	-DCMAKE_CXX_COMPILER:FILEPATH=${CMAKE_CXX_COMPILER}
	-DCMAKE_CXX_FLAGS:STRING=${CMAKE_CXX_FLAGS}
	-DCMAKE_C_COMPILER:FILEPATH=${CMAKE_C_COMPILER}
	-DCMAKE_C_FLAGS:STRING=${CMAKE_C_FLAGS}
	-DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}
	)

[jcfr]

See https://kripken.github.io/talks/wasm.html and https://github.com/WebAssembly/binaryen

[thewtex]

@jcfr The option is -s WASM=1.

I would recommend using itk.js. Among the benefits, it includes:

1. Building both an asm.js and wasm module for you and using the asm.js on browsers that do not have wasm support transparently.
2. Bundles and locates files so you do not have difficulty finding WASM assets (you will run into this issues if you try doing it without itk.js, and you will run into more issue if you try using the module outside of the dcm.js example).
3. Execution in a web worker to improve interactivity in the main thread.

[jcfr]

It would be neat to levrage itk-js infrastructure, are you suggesting we update itk-js project so that we can reuse its infrastructure ? Or adding dcmjs as an itk-js plugin ?

[thewtex]

@jcfr itk.js could be used as-is to perform the dcm.js build. The steps are

1. Set up to use the insighttoolkit/itk-js dockcross image for the build.
2. Slight tweaks to how the module execution is invoked.
3. Maybe something else, but I will be happy to help if there are any issues :-)